One of the biggest challenges faced by river engineers today is stabilizing degrading channels. Urbanization and the associated increase in the amount of impervious area have caused an increase in runoff and stream flow rates. As a result, many stream beds have experienced a reduction in channel slope through downcutting or increased channel meander. For this reason, a variety of structures have been installed in streams to minimize channel degradation. The following blog post provides an overview of drop structure design and how to model drop structures in HEC-RAS.
What are drop structures?
Drop structures, which are also referred to as grade control structures, are devices installed for the purpose of mitigating the long-term degradation of channels. This is important because channel degradation can cause damage to bridges, culverts, and utility lines. Bank erosion and loss of riparian habitat are also undesirable consequences of excessive channel degradation. River engineering projects install drop structures because they can serve as an alternative to channel linings, and they have a relatively small footprint. Unfortunately, they can serve as a barrier to fish passage, and there can be permitting issues associated with installing drop structures in regulatory waterways. Fish ladders can be added to grade control structures to make them more “fish-friendly.”
It is important to note that drop structures can be a safety hazard. For this reason, the vertical drop of grade control structures is limited in some areas. Many agencies limit the vertical drop of a grade control structure to five feet.
There are several types of drop structures. They can be exposed or buried, depending on the design. Some examples of grade control structures include:
- Grouted rock grade control structures,
- Reinforced concrete grade control structures,
- Gabion grade control structures,
- Sheet pile grade control structures,
- Loose rock structures,
- Soil cement grade control structures,
- Boulder weir structures, and
- Regenerative stormwater conveyance (RSC).
Throughout my career, I have been involved in several projects that included drop structure design. Most of these projects were “traditional” concrete and grouted rock grade control structures. In the future, I hope to see more regenerative stormwater conveyance (RSC) projects because, in addition to mitigating channel degradation, RSCs promote infiltration and provide stormwater treatment.
Step 1: Equilibrium Slope Analysis
The purpose of installing drop structures/grade control structures is to establish a fixed elevation in a stream bed to limit potential streambed degradation by encouraging the stream to degrade towards the proposed “equilibrium slope.” The equilibrium slope is the channel slope where sediment supply equals sediment degradation within a particular reach of a stream. Thus, a series of properly designed grade control structures aid in adjusting the sediment transport capacity to equal sediment supply.
Determining equilibrium slope is an iterative process that begins with developing a one-dimensional (1D) hydraulic model of the channel being analyzed. After developing a hydraulic model, the first step of an equilibrium slope analysis is to determine reaches with similar hydraulic characteristics and to identify a “control point” where no sediment aggradation or degradation will occur. Next, analyze a portion of the channel upstream of the reach of interest to calculate the sediment supply (tons/year). Then develop a new geometry file where the reach of interest is sloped at a possible equilibrium slope (I recommend starting at 0.01). Determine the sediment transport capacity (tons/year) of the reach of interest at that slope. Repeat this process until the sediment transport capacity of the reach of interest is close, ideally equal to the sediment supply.
I have used the SAM Hydraulic Design Package for Channels (USACE, 1998), which was developed by the United States Army Corps of Engineers Waterways Experiment Station, to determine sediment transport capacity. HEC-RAS has Stable Channel Design Functions that are based on SAM.
Step 2: Drop structure placement
Drop structure placement is another iterative placement. The image below provides a visual of how drop structure placement works. The spacing between drop structures is based on height and the equilibrium slope determined in Step 1. The height of the drop structure depends on the type of structure, local regulations, and hydraulic/scour criteria. Drop structure placement is an art that involves balancing the number of drop structures, which can be expensive to install, with the height of each drop structure.
Step 3: Local Scour Analysis
After placing each drop structure, estimate the amount of local scour at the downstream end of each grade control structure. There are a variety of methods available for estimating local scour. The scour depth below drop structures can be estimated using the methodology outlined by Bormann and Julien (1991). Scour downstream of a vertical structure can also be estimated using the Veronese method (USBR).
Step 4: Modeling Drop Structures in HEC-RAS
The United States Army Corps of Engineers (USACE) Hydrologic Engineering Center (HEC) has conducted tests that verify that HEC-RAS can be used to model drop structures. This verification study involved comparing the results of physical tests for a large drop structure along the Santa Ana River in Orange County, California to HEC-RAS model results (USACE, 1994h). Water surface profiles generated by the HEC-RAS model closely matched the results of the physical test.
Modeling drop structures using cross-sections
When modeling drop structures using cross-sections, it is important to place sections at close intervals to ensure the location of the hydraulic jump, which is the transition from subcritical flow to supercritical flow, is accurately determined. It is important to run your model in the mixed flow regime when modeling drop structures using cross-sections. It should be noted that Manning’s roughness coefficient (n) values should be increased inside of a stilling basin to account for the significant loss of energy due to the hydraulic jump. For this reason, selecting a Manning’s n that only represents the structure’s surface roughness will not result in an accurate water surface profile.
Modeling drop structures as an inline weir
If you are not concerned with generating a detailed water surface profile and identifying the exact location of the hydraulic jump through a drop structure, using an inline weir to represent a drop structure is a good alternative. Inline weirs are usually larger structures spanning across major streams and rivers while drop structures are typically located along smaller creeks and streams. For this reason, it is important that the modeler selects an appropriate weir coefficient and accurately represent the drop structure geometry.